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AGRONOMY
GETTING THE DIRT ON DIRT
It may look like just a speck of dirt to the naked eye, but under an electron microscope a crumb of prairie soil is really a carefully constructed "apartment building," home to the small critters that recycle decaying organic matter into usable nutrients. About a millimeter across, a soil crumb--or macroaggregate--is riddled with water- and air-filled pores that shelter such organisms as bacteria, fungi and nematodes. As these organisms dine on dead roots, fertilizer and even one another, they release the nitrogen compounds that feed growing plants.
U.S. Department of Agriculture soil scientist Cynthia Cambardella is passionate about macroaggregates. She and her colleagues at the National Soil Tilth Laboratory in Ames, Iowa, study soil structure and its effect on nutrient cycling in the hope of developing more efficient and environmentally friendly farming techniques. Soils with abundant macroaggregates do a better job of supporting plant life and lose fewer nutrients to leaching; therefore, much of Cambardella's work focuses on the formation and degradation of these rich crumbs in agricultural lands.
Cambardella has her graduate student Jeff Gale of Iowa State University use radioactive carbon 14, for example, to observe the aggregation process in soil as the debris from harvested oat plants decomposes. Gale sows his oats in large pots kept in a walk-in growth chamber. As the plants grow to a height of a meter, he periodically doses them with radioactivity by combining carbon 14-tagged sodium bicarbonate with a dilute acid inside the growth chamber. When the acid hits the radioactive baking soda, the mixture foams, liberating "hot" carbon dioxide that is fixed by the growing plants and incorporated into their tissues.
After the plants are harvested, Gale and Cambardella can trace the radioactive organic materials from the remaining roots and straw as they decompose and become incorporated into the soil structure. They are finding that the presence of relatively fresh plant matter in the soil helps to stabilize its structure--the number of macroaggregates peaks about 180 days after the harvest, and then the aggregates start to break down, potentially compromising soil quality.
Studying aggregate formation, Gale says, will help farmers learn to maintain good soil structure in the field. Cambardella believes that understanding soil structure is vital to developing agricultural practices that do not cause topsoil erosion or the contamination of aquifers and surface waters with fertilizer runoff. "We need to learn more about what's really going on in the soil," Cambardella says. "We can't black-box it anymore." --Rebecca Zacks
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